update cython to 3.1.0a1

Author: mattip

.github/workflows/pythran.yml

@@ -45,6 +45,7 @@ jobs:
       run: |
         cd pythran
         python -m pip install --upgrade pip
+        python -m pip install "cython==3.1.0a1"
         python -m pip install "numpy==2.2.0rc1" -Csetup-args="-Dallow-noblas=true"
         pip install -r requirements.txt
         pip install ipython nbval pytest-xdist wheel